sauttefk / RS485HwSerial

Enhanced Arduino serial library with RS485 transmit enable control

The RS485HwSerial library is an enhanced version of the standard Arduino serial functions with control for RS485 transmit enable.
It automatically enables the transmit enable pin TE on half-duplex RS485 transceivers when a character has to be sent and disables the TE pin when the last stop bit was sent TXC, by utilizing the USART transmit complete interrupt USART_TXC_vect.
The TE signal is only active (and therefore the transmitter blocking the bus) as long as it is really needed.
Half-duplex RS485 communication can be achieved non-blocking

This is a major improvement over

• hardware solutions that use monoflops to control RS485 TE triggered by the TX-signal
• software solutions that are (blocking) waiting for the transmit buffer to be empty and another extra delay for the last byte to be shifted out

Alternative hardware solution

Instead of using a microcontroller based solution for controlling the TE signal like this or any other RS485 library, you may use a RS485 transceiver with auto direction control like the MAX13487E.

Installation

• Download the RS485HwSerial master zip file.
• Extract all files into a RS485HwSerial subdirectory in your Arduino/libraries directory.

Usage

#include <RS485HwSerial.h>

// RS485 transceiver's transmit enable pin (often called TE or DE).
// Can be tied together with negated receive enable pin to disable local echo
// (often called /RE). This pin can freely be chosen.
#define RS485_TE  13

// RX pin of serial 1 on ATmega328PB. if /RE on RS485 transceiver is used then
// there should be pull-up resistor on RX or a least a weak pull-up configured on
// the RX pin (see below), as the RO pin of the RS485 transceiver becomes high
// impedance if the receiver is disabled.
#define RS485_RX  12

// Baud rate/BPS on RS485 port
#define RS485_BPS  9600


void setup() {
  // rs485 transceiver transmitter enable pin
  RS485HwSerial1.transmitterEnable(RS485_TE);
  // rs485 uart setup
  RS485HwSerial1.begin(RS485_BPS);

  // enable weak pull-up on RX pin (better use pull-up resistor)
  pinMode(RS485_RX, INPUT_PULLUP);
}


void loop() {
  RS485HwSerial1.println(F("\nHello Serial1 RS485"));
  delay (1000);
}

Illustration of the TE signal and half-duplex usage of the RS458-bus

      _____ __ _..._ __ _____________..._______________
RXD:       X__X_..._X__X

      __________..._______ __ __ __ _..._ __ __________
TXD:                      X__X__X__X_..._X__X  

                         ____________...______
TE:   __________..._____|                     |________

      __________...______                 _____________
UDRE:                    ||__|__|__|_..._|

      __________...______                     _________
TXC:                     |___________..._____|

      -----RX---...-----|-------TX---...------|---RX---

RXD: USART Receive Data
TXD: USART Transmit Data
TE: Transceiver Transmit Enable
UDRE: USART Data Register Empty
TXC: USART Transmit Complete

Connecting RS485 Transceiver

ASCII schematic of connecting a half-duplex RS485 transceiver e.g. MAX487CSA+ to a microcontroller.
Shielded twisted pair network cable is recommended. E.g. S/UTP, F/UTP, SF/UTP

----+      * Vcc
    |      |
AVR |     | | 47k     /|             * Vcc
    |     | |        / | A           |
    |     | |       /  +-------+    | | 560R
    |      |   RO  /   |       |    | | bias pull-up
 RX +------*------+    |       |    | | (only on master)
    |              \   | B     |     |
    |           /RE o  o---+   *-----*-------------------------- RS485 A/+
    |               |\ |   |   |     |
    |               | \|   |   |     |
 TE +---------------*      |   |    | | 120R
    |               |      |   |    | | termination resistor
    |             |\|      |   |    | | (on each bus end)
    |             | + TE   |   |     |
    |             |  \   B |   |     |
    |          DI |   o----*---|-----*-------------------------- RS485 B/-
 TX +-------------+    \       |     |
    |             |     +      |    | | 560R                +--- cableshield
    |             |    / A     |    | | bias pull-down      |
    |             |   +--------+    | | (only on master)   | | 100R
    |             |  /               |                     | | 1W
    |             |/                 * GND                 | |
----+                                                       |
                                                            * GND           

Excellent information about the electrical aspects of RS485 like proper termination, biasing and shielding can be found here:
http://support.fccps.cz/download/adv/frr/RS485/RS485.html
This covers additionally the aspects of ESD-protection:
https://www.renesas.com/eu/en/www/doc/whitepapers/interface/rs-485-transceiver-tutorial.pdf
https://www.renesas.com/eu/en/www/doc/whitepapers/interface/rs-485-transceiver-tutorial_de.pdf

Author

• Frank Sautter
• Based on the Arduino hardware serial library of Nicholas Zambetti